The following patent publications are believed to represent the current state of the art:
U.S. Pat. Nos. 6,351,767; 6,408,360; 6,757,708; 6,823,374; 7,096,418; 7,320,023; 7,343,412; and
U.S. Published Patent Application Nos.: 2002/0120710; 2003/0004998; 2004/90044731; 2005/0240732; 2009/0049243.